1. Field of the Invention
The present invention relates to the removal of nitrogen oxides from gas streams comprising same, and, more especially, to the catalytic reduction of the nitrogen oxides in nitrogenous gases by means of ammonia, and whereby a gas stream essentially completely freed of nitrogen oxides (NO.sub.x) results.
2. Description of the Prior Art
The synthesis of nitric acid via the oxygen oxidation of ammonia has long been known to this art. The initial reaction in the synthesis is conducted at elevated temperature and yields nitric oxide, NO, which is then itself oxidized with oxygen at a lower temperature to yield nitrogen dioxide, NO.sub.2. This NO.sub.2 is next in turn reacted with water in a dismutation reaction yielding nitric acid and nitric oxide in oxido/absorption columns. The nitric oxide is next in turn "reoxidized" by the residual oxygen and the process continues by dismutation.
A conventional nitric acid production plant typically comprises two absorption columns. In the second column, however, the partial pressure of the oxygen and the partial pressures of the nitrogen oxides (NO and NO.sub.2) in the gaseous flow are such that the oxidation and absorption processes are slowed to the point where it would be necessary to markedly increase the number of plates in the column to eliminate the nitrogen oxides which are not converted to nitric acid and which are present in the residual gas flow. For this reason, in order to avoid exceedingly costly capital investment, the residual gases are discharged to the atmosphere and which gases comprise appreciable amounts of the nitrogen oxides, amounts of between 1,000 and 10,000 vpm. Such discharges, however, obviously present the serious problem of pollution of the environment. Consequently, various national laws recently promulgated required that such residual gases contain substantially lower amounts of the oxides of nitrogen (NO.sub.x). Thus, the French standard currently mandates a discharge of less than 400 vpm, and the USA standard, a discharge of less than 255 vpm (vpm=volume per million)
For the foregoing reason, numerous processes have been proposed to reduce the NO.sub.x content of residual gaseous flows to permissible values.
It, too, has long been known to this art to catalytically reduce the content in nitrogen oxides of a gas stream containing oxygen, by means of ammonia in the presence of the platinum group metals; compare, for example, French Patent No. 1,205,311. However, at those temperatures necessary for such reduction, typically higher than 250.degree. C., the catalyst rapidly loses its activity and at lower temperatures this activity is inadequate.
It is also known to the art to catalytically reduce nitrogen oxides with a gaseous stream of ammonia in the presence of a catalyst based on vanadium oxide (V.sub.2 O.sub.5) borne by a porous support. Typically, this process requires a given NH.sub.3 /NO.sub.x molecular ratio (usually about 1) and for high hourly space velocities (VVH) on the order of 20,000 (VVH=volume of gas per volume of catalyst per hour) an inlet temperature for the gas to be purified and that is relatively high is required, typically in excess of 250.degree. C., to obtain acceptable NO.sub.x conversion rates compatible with a substantial purification of the flow of the gas.
In the event that the temperature of the gas to be purified is less than 250.degree. C., for example, about 200.degree. C., the activity of the known catalysts, V.sub.2 O.sub.5 /Al.sub.2 O.sub.3, is insufficient to afford acceptable pollution reduction rates, such as those dictated by the standards mentioned hereinabove, without supplementary heating and a substantial reduction in space velocity. Raising the temperature of the gas to be treated, for example, from 200.degree. C. to 250.degree. C., entails additional energy expenditures and, further, necessitates a reduction in the hourly space velocity, resulting in still additional investment expenditures due to the increase in the volume of the reactor.
It will thus be seen that there exists a significant need in this art for a catalyst and a pollution abatement process vis-a-vis the NO.sub.x contained in a gas flow, and desirably based upon an ammonia process affording direct treatment of the gaseous effluent, for example, from the absorption columns of a nitric acid production facility, and entailing a temperature of less than about 250.degree. C. without the necessity for preliminarily reheating the gas, the reaction being effected in a reactor of the smallest possible volume in order to minimize the capital investment required.